Smart objects can be connected to the Internet and communicate with each other, either using wired or wireless connections, to form an “Internet of Things.” The smart objects can include, for example, phones, personal desktop or laptop computers, tablet computers, refrigerators, and many other items. By enabling objects to communicate with one another, the objects may be able to determine what users like, want, and/or need, and act accordingly, potentially improving the quality of people's lives. The term “Internet of Things” was firstly proposed by Kevin Ashton in his presentation at Procter & Gamble in 1999. During the presentation, Ashton envisioned the potential of Internet of Things by stating “The Internet of Things has the potential to change the world, just as the Internet did. Maybe even more so.” In 2005, the Internet of Things was introduced by the International Telecommunication Union (ITU) through the ITU Internet report.
Current wireless technologies include two groups: 1) wireless technologies for low-data-rate and low-power applications such as remote control, and 2) wireless technologies for high data rate applications such as video streaming. The technologies suitable for low data rate applications may not be able to meet the requirements of the high data rate applications. For example, a wireless communication technology suitable for low power, low data rate applications is ZigBee. Mainly based on IEEE 802.15.4, ZigBee can operate in the 868 MHz, 915 MHz and 2.4 GHz bands with respective data rates of 20 kb/s, 40 kb/s and 250 kb/s. A similar technology is Z-Wave, whose main purpose is to enable short message transmission from a control node to multiple nodes. The maximum speed of Z-Wave is 200 kb/s working at 2.4 GHz band. An advantage of ZigBee and Z-Wave is the low price. Both of these technologies are designed for low-power applications in battery-operated devices. Moreover, ZigBee includes a sleep mode mechanism to reduce power consumption. The complexity of hardware is low: 32-128 kbytes of memory is enough to implement the system including the higher layers. The disadvantage of ZigBee and Z-Wave is their low data rate. Moreover, the 2.4 GHz frequency band is crowded with interfering devices, e.g., microwave ovens, WiFi equipment, and cordless phones. The sub-GHz electromagnetic (EM) waves propagate very far, so very high node density may not be achievable due to the high interference levels generated by other similar devices.
Technologies for high data rate applications include Bluetooth and WiFi. Bluetooth, based on IEEE 802.15.1, is a wireless technology for exchanging data over short distances. Compared with ZigBee and Z-Wave, the data rate can be increased to Megabit per second (Mbps). WiFi, based on IEEE 802.11, allows an electronic device to exchange data or connect to the Internet wirelessly. The speed of WiFi can be up to several Gigabit per second (Gbps) according to IEEE 802.11ac with the help of multiple-in-multiple-out (MIMO) technology and high order modulation. The advantage of these two technologies is the high data rate. However, they require higher power consumption, higher complexity of hardware (MIMO in WiFi), and thus higher price. Because both the transmitter and the receiver use the same architecture, i.e., symmetric architecture is used, the power consumption of terminal devices is high. In addition, a large number of WiFi access points (APs) deployed close to each other operating in the same or adjacent channels may interfere with each other. Another wireless technology is the 3G/4G mobile communications. However, the indoor coverage of 3G/4G signals may be poor.